1. Field of the Invention
The present invention relates to a semiconductor light emission system, and is particularly concerned with a semiconductor laser capable of controlling a radiant power on a small current.
2. Description of the Prior Art
FIG. 7 is a characteristic curve representing one example of current vs. radiant power characteristic according to a conventional semiconductor light emission system, and FIG. 8 is a sectional perspective diagram representing a prior art semiconductor light emission system mentioned, for example, in Japanese Patent Laid-Open No. 192380/1985.
In FIG. 7, the quadrature axis indicates an operating current, and the axis of ordinate indicates a radiant power.
In FIG. 8, (1) denotes a first conductive (p-type or hereinafter referred to as "p-") GaAs substrate, (2) denotes a p-Al.sub.0.45 Ga.sub.0.55 As buffer layer, (3) denotes a second conductive (n-type or hereinafter referred to as "n-") GaAs current block layer which is a current restricting layer with a striped groove provided on the p-GaAs substrate (1), (4) denotes a p-Al.sub.0.45 Ga.sub.0.55 As lower clad layer, (5) denotes a p-type or n-type Al.sub.0.15 Ga.sub.0.85 As active layer, (6) denotes an n-Al.sub.0.45 Ga.sub.0.55 As upper clad layer, (7) denotes an n-GaAs contact layer, (8) denotes a p side electrode, and (9) denotes an n side electrode.
A function of the prior art semiconductor light emission system will be described with reference to FIG. 7 and FIG. 8.
When a positive voltage is impressed on the p side electrode (8) and a negative voltage on the n side electrode (9), a junction between the n-GaAs block layer (3) and the p-Al.sub.0.45 Ga.sub.0.55 As lower clad layer (4) becomes inverted of bias in a domain where the n-GaAs block layer (3) is present between the electrodes (8), (9), therefore a current does not flow therein, and the current flows in concentration only on the opening of a striped groove (5a) or thereabout. In this case, a hole is injected into a portion near a bottom of the striped groove (5a) of the Al.sub.0.15 Ga.sub.0.85 As active layer (5) from the p-Al.sub.0.45 Ga.sub.0.55 As lower clad layer (4), an electron is injected then from the n-Al.sub.0.45 Ga.sub.0.55 As upper clad layer (6), and thus a light is emitted through a recombination of both the two. According as a current level to be injected is further raised, an inductive emission begins presently, as shown in the characteristic curve of FIG. 7, to lead to a laser oscillation. A threshold current for laser oscillation will be 20 to 30 mA in the case of the semiconductor light emission system of FIG. 8, and an operating current for obtaining a 3 mW radiant power is required at 40 mA or so.
The prior art semiconductor light emission system has the above-described construction, and hence when a radiant power is to be controlled, a laser driving current must be controlled direct, and since the driving current is large to run from several tens mA to several hundreds mA, the problem is that a control circuit becomes inevitably complicated. Additional prior art is disclosed in the article entitled: "A Novel Self-Aligned Laser with Small Astigmatism Grown by MO-CVD" by Y. Mihashi, Y. Nagai, H. Matsubara and K. Ikeda, which is shown on extended abstract of the 17th Conference on Solid State Devices and Materials, Tokyo, pp. 63-66.